Click either 'Choose when to turn off the display', 'Change when the computer sleeps', or 'Change plan settings' on whatever power plan you're on. Click the little battery icon in the lower right-hand corner of the screen Here's an exact step by step of what I did:

Hello! I just fixed that exact problem that's been bugging me for months, so I wanna get the fix out there for others. If trouble persists you may want to try updating the BIOS, I personally found that Win 8.1 doesn't really like the older versionsВ andВ all theВ updates can be obtained from the HP website. The other things to be suggested is looking atВ the screen resolution settings, sometimes they may need a slight tweak and of course theВ Graphic card settings and drivers, PersonallyВ I would just set all theВ major settings forВ itД«ack to default (most are computer decided anyway)В and make sure all the driversВ are up to date for bothВ the card and monitor as well. After making the changes click 'Apply' then 'Ok'. Then further down the listВ find 'Display'ДЄnd check on the 'Adaptive brightness' making sure they aren't different either. While still in the power menu click 'Change advanced power settings' and look at your Intel graphic settings to make sure they are on 'Maximum performance' or at least are both the same for plugged and otherwise. In and an eco setting when you unplug has a marked difference.

There you can check the screen brightness setting for both 'plugged in' and 'unplugged' also check the actual power plan that you're set to. Continuing what was already said, Yes 1st stop should be the power settingsВ menu.

Laptop dims when plugged in